The present invention relates to an apparatus and a method of measurement used for evaluating the remanence magnetization curve (hysteresis characteristic, residual magnetization characteristic, etc.) of a magnetic material (magnetic recording medium) such as a magnetic disk device. More particularly this invention relates to an apparatus and a method of measurement used for evaluating the characteristics of a super-high density magnetic material.
The recording density of the magnetic disk device thus far used as an external storage unit of the computer has increased at a surprising rate of 100 times for each decade. Critical factors that have contributed to such an increase in the recording density at this rate are improvement in the data transfer rate, reduction in the thickness of the magnetic material, reduction in the size of crystal particles constituting the magnetic material, etc. With the recent progress in reduction in the thickness of the magnetic material and the size of crystal particles, the ever decreasing volume of the magnetic members has caused an unstable direction of magnetization under the thermal effect and easy disappearance of the recording data. This phenomenon of thermomagnetic relaxation has cropped up as a problem constituting a bottleneck to a higher density.
Further, with the increase in the data transfer rate, the problem of an increased effective coercive force of the magnetic material has come up, giving rise to the fear of degradation in the overwriting characteristic. If only to overcome this problem in developing a magnetic disk device, an apparatus has been required for measuring and evaluating the remanence magnetization curve of a magnetic material accurately in accordance with practical conditions.
A magnetic material remanence curve measuring apparatus that evaluates the remanence magnetization courve (hysteresis characteristic, residual magnetization, etc.) of the magnetic material during the development of a magnetic disk apparatus is know in the prior art. By feeding back the evaluation result obtained by such a magnetic material remanence curve measuring apparatus, the remanence magnetization courve of the magnetic material are improved in such a way that they meet the high density requirement. Thus, the development of the magnetic material remanence curve measuring apparatus plays a very important role as a measuring instrument aimed at a higher density.
Methods of this type for measuring the remanence curve of the magnetic material include the VSM (Vibrating Sample Magnetometer), an electromagnetic induction method using the SQUID (Superconducting QUantum Interference Device) well known as a superconductive quantum interferometer, the Faraday method using AGM (Alternative Gradient Magnetometer) and a method using the magneto-optical effect (Kerr effect). In the VSM, a sample (a magnetic material, for example) placed and magnetized in the air gap between an electromagnet is vibrated vertically with a small amplitude at a predetermined frequency, and the magnetic field generated by this magnetized sample is used to generate vibration magnetic fluxes in a search coil arranged in the neighborhood of the sample. Then, the magnetization is measured based on an AC signal generated in the search coil. Magnetization is measured while exerting an external magnetic field statically changing over a very long time of 10 to 20 minutes on the sample.
The SQUID, on the other hand, is a high-sensitivity magnetic sensor capable of detecting a very weak magnetic field equivalent to not more than one 50 millionth of the Earth""s magnetic field by use of the superconductive quantization. This device is used in an environment maintained at a very low temperature utilizing liquid helium or the like. Specifically, this SQUID is a device formed by micromachining a superconductive thin film so that weak superconductive junctions are parallel to each other. The voltage across the device undergoes a change as a magnetic field is applied thereto from a sample with the bias current fixed at about the critical current value. By grasping this voltage change, the strength of the magnetic field can be measured.
Further, the Kerr effect measuring apparatus that measures the remanence magnetization curve using the Kerr effect described above is disclosed in Japanese Patent Application Laid-Open No. SHO 63-122930. This apparatus comprises light condensing means for condensing a polarized laser beam and radiating it on a magnetic material, magnetic field generating means f or applying a magnetic field to a magnetic material, and means for detecting the polarized state of the laser beam reflected from the magnetic material. In this Kerr effect measuring apparatus, the remanence magnetization curve of the magnetic material is measured taking advantage of the fact that the rotational angle of the polarization plane of the reflected laser beam detected by the detecting means when the magnetic field of the magnetic field generating means is changed undergoes a change in accordance with the direction in which the magnetic material is magnetized.
Still another apparatus for measuring the remanence curve of a magnetic material is a pulse magnetic field application and measuring apparatus. This apparatus comprises a coil for generating a pulse magnetic field, a power supply for supplying power to the pulse magnetic field generating coil, a detection coil arranged at the central potion of the pulse magnetic field generating coil, and a measuring unit for measuring the remanence magnetization curve of the sample disposed in the neighborhood of the detection coil based on the eIectromotive force generated in the detection coil.
In the prior art, a magnetic material has been developed, which has a remanence magnetization curve capable of standing a high density recording by evaluating the characteristics of a sample based on the measurements obtained from the various apparatuses described above for measuring the remanence curve of a magnetic material. Specifically, the film thickness and the size of crystal particles are reduced, and the thermal dependency of the coercive force in the magnetic material is improved. In the magnetic material remanence curve measuring apparatuses other than the pulse magnetic field application and measuring apparatus described above, the write operation by the magnetic head of the magnetic disk device is simulated by applying a magnetic field to the sample that changes at low speed over a time span of 10 to 20 minutes and the remanence magnetization curve of the sample are measured.
In the actual magnetic disk device, the rate of change of the magnetic field of the magnetic head of the magnetic material reaches the order of as high as several MHz to several tens of MHz due to the greatly increased data transfer rate constituting an important factor of high density recording. In the various conventional apparatuses for measuring the remanence curve of the magnetic material (except for the pulse magnetic field application and measuring apparatus), on the other hand, the change rate of the magnetic field applied to the sample is in the order of 10 to 20 minutes, which is considerably different from the actual change rate (several MHz to several tens of MHz) of the magnetic field of the magnetic disk device.
Specifically, in the conventional magnetic material remanence curve measuring apparatus, the conditions under which the magnetic field is applied to the sample fail to meet practical requirements. With the recent progress of reduction in the thickness of a magnetic film and the size of magnetic particles in the magnetic material, the problem has cropped up that the resulting remanence magnetization curve are considerably different from the remanence magnetization courve for practical applications. As a specific example, the measurement of 2500 (Oe) was obtained as a coercive force of a sample by the VSM in a recently developed magnetic material. The effective coercive force for practical applications, however, is something about 5000 (Oe) (theoretical value). Thus, the difference (measurement error) is as large as 2500 (Oe).
Assume that a magnetic material having an apparent coercive force of 2500 (Oe) is built in a magnetic disk device believing in the measurement of the apparatus for measuring the remanence curve of the magnetic material described above. The magnetic head will be designed to overwrite the magnetic material in an applied magnetic field of at least 5000 (Oe) of coercive force. In view of the fact that the actual coercive force of the magnetic material is 5000 (Oe), however, the very serious situation occurs that the overwrite operation is impossible even when the magnetic field of 5000 (Oe) is applied to the magnetic material at the magnetic head. In other words, in the conventional apparatus for measuring the remanence curve of the magnetic material (except for the pulse magnetic field application and measuring apparatus), a disagreement develops between the measurement of the remanence magnetization curve and the remanence magnetization curve for practical applications. This poses a very serious problem that the overwrite characteristic is deteriorated by the increased effective coercive force.
In the pulse magnetic field application measuring apparatus, on the other hand, the magnetic field can be changed with rapidity by the pulse magnetic field. The resulting advantage is that a rapidly-changing magnetic field can be exerted on the sample under conditions similar to practical applications. Nevertheless, in view of the fact that the electromotive force generated in the detection coil is detected by use of the electromagnetic induction, a minor magnetic moment such as that of a magnetic material cannot be detected. Thus, the pulse magnetic field application measuring apparatus, though effective for the sample having a large magnetic moment, is not suitable for measuring the remanence magnetization curve of the magnetic material of which the reduction in thickness is going on.
The present invention has been developed in view of the points described above. It is the object of the present invention is to provide an apparatus and a method for measuring the remanence curve of a magnetic material with high accuracy under measurement conditions meeting practical requirements.
In the first aspect of the present invention, a pulse magnetic field is applied to the magnetic material and rapidly changed with the help of a pulse magnetic field applying unit. This is very similar to the state in which the magnetic field is generated by the magnetic head with the magnetic material built in a magnetic disk device. After the pulse magnetic field is applied, the remanence magnetization courve are measured by a measuring unit based on the principle of detecting the force that the magnetic moment receives from the magnetic field gradient. As compared with the conventional detection coil, therefore, a high sensitivity is obtained even for a magnetic material having a very small magnetic moment. In this way, according to the first aspect of the present invention, the measurement meeting the practical conditions can be obtained with a higher accuracy.
In the present invention according to the second aspect, a pulse magnetic field is applied to the magnetic material and rapidly changed with the help of a pulse magnetic field applying unit and the remanence magnetization curve are measured by a measuring unit based on the detection result of the polarized state of the light utilizing the Kerr effect. This case is also similar to the situation in which the magnetic field is generated by the magnetic head with the magnetic material built in a magnetic disk device. Further, a high sensitivity as compared with the conventional detection coil is secured even with a magnetic material having a very small magnetic moment. In this way, according to the second aspect of the present invention, the measurements meeting practical requirements can be obtained with a higher accuracy.
In the present invention according to the third aspect, the pulse magnetic field that changes as rapidly as 100 ns to 100 ms is applied with the help of a pulse magnetic field applying unit. As a result, the remanence magnetization courve can be measured under the conditions very similar to the practical conditions of a hard disk device or the like.
In the fourth aspect of the present invention, the angle of arrangement of the magnetic material is adjusted to arbitrary degrees with the help of a angle adjusting unit. Therefore, an actual head magnetic field having an angular component can be produced in simulation, and thus not only conditions meeting practical applications can be produced but also variations of measuring methods can be increased.
In the fifth aspect of the present invention, a space having a zero magnetic field and a finite magnetic field gradient is generated with the help of a magnetic field gradient generating unit. Therefore, the residual magnetization (remanence magnetization curve) of the magnetic material after application of the pulse magnetic field can be measured very accurately.
In the sixth aspect of the present invention, a steady magnetic field is exerted on the magnetic material with the help of a steady magnetic field generating unit, and therefore the remanence magnetization curve of the magnetic material in the steady magnetic field (external magnetic field) can be measured for an improved general purpose applicability. Further, in the sixth aspect of the present invention, the change of the remanence magnetization curve in the steady magnetic field is measured by use of the Kerr effect.
In the eighth aspect of the present invention, a pulse magnetic field is applied to the magnetic material and rapidly changed. This situation is very similar to the one in which a magnetic field is generated by a magnetic head with the magnetic material built in the magnetic disk device. In the measuring process after application of the pulse magnetic field, the remanence magnetization courve are measured according to the principle of detecting the force received by the magnetic moment from the magnetic field gradient. As compared with the conventional detection coil, therefore, a high sensitivity is obtained even with a magnetic material having a very small magnetic moment. In this way, according to the present invention of the eighth aspect, the measurements meeting practical conditions can be secured for an improved accuracy of the measurements.
In the ninth aspect of the present invention, a pulse magnetic field is applied to the magnetic material and rapidly changed, further, the remanence magnetization curve is measured based on the detection result of the polarized state of the light utilizing the Kerr effect. This case is also similar to the situation in which the magnetic field is generated by the magnetic head with the magnetic material built in a magnetic disk device. Further, a high sensitivity as compared with the conventional detection coil is secured even with a magnetic material having a very small magnetic moment. In this way, according to the ninth aspect of the present invention, the measurements meeting practical conditions can be secured with a higher measurement accuracy.
In the tenth aspect of the present invention, the space of a zero magnetic field and a finite magnetic field gradient is generated, and therefore the residual magnetization (remanence magnetization curve) in the magnetic material after application of a pulse magnetic field can be measured very accurately.
In the eleventh aspect of the present invention, a steady magnetic field is exerted on the magnetic material, and therefore the remanence magnetization courve of the magnetic material in the steady magnetic field (external magnetic field) can also be measured, thereby improving the general purpose applicability.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.